How to get term position in sort script

In Elasticsearch 8, I have the following index:

{
    "settings": {
        "analysis": {
            "analyzer": {
                "default": {
                    "type": "whitespace"
                },
                "default_search": {
                    "type": "whitespace"
                }
            }
        }
    },
    "mappings": {
        "properties": {
            "title": {
                "type": "text",
                "term_vector": "with_positions"
            }
        }
    }
}

And this query

{
    "query": {
        "simple_query_string": {
            "query": "banana"
        }
    },
    "track_scores": true,
    "sort": {
        "_script": {
            "type": "number",
            "order": "desc",
            "script": {
                "source": " >>> HOW TO GET THE POSITION OF banana IN THE title FIELD? <<< ",
                "lang": "painless"
            }
        }
    }
}

I found very old answers, such as:
_index['title'].get('banana',_POSITIONS);
But that errors:
cannot resolve symbol [_index]

I need this because I want higher score for documents where the query appears earlier in the title field.